If it's a war which breaks the term of the United Nations Charter, which an attack on Iraq will do, unless the Security Council backs it (which seems highly unlkely at this stage), then it depends on what status that has in the legal system of the country involved.

I'd think that in the case of the UK it would be illegal. A court refused to rule on this when an action was brought by CND last year, largely I think because it was a hypothetical case.

I'd have thought the same would be true in the USA, since it involves a solemn treaty obligation.

But I'd imagine, whatever the actual theoretical legal situation, any court is going to roll over on an issue like this.